Method for manufacturing solar cell and solar cell

ABSTRACT

The present invention provides a method for producing a solar cell comprising forming the solar cell from a CZ silicon single crystal wafer, wherein a CZ silicon single crystal wafer having an initial interstitial oxygen concentration of 15 ppma or less is used as the CZ silicon single crystal wafer; a solar cell produced from a CZ silicon single crystal wafer, wherein the CZ silicon single crystal wafer has an interstitial oxygen concentration of 15 ppma or less; and a solar cell produced from a CZ silicon single crystal wafer, wherein the CZ silicon single crystal wafer has a BMD density of 5×10 8 /cm 3  or less. Thus, there can be obtained a solar cell showing little fluctuation of characteristics.

TECHNICAL FIELD

[0001] The present invention relates to a method for producing a solarcell utilizing a silicon single crystal wafer useful as a material ofsolar cell and a solar cell.

BACKGROUND ART

[0002] When a silicon single crystal is used as a material for producinga solar cell, reduction of the production cost as well as improvement ofthe conversion efficiency have constituted serious problems.

[0003] Hereafter, the technical background of use of a silicon singlecrystal as a material for solar cells will be explained.

[0004] Characteristics of solar cells will be explained first based ontype of a substrate material constituting a solar cell. Solar cells areroughly classified based on the type of the substrate material intothree types, i.e., “silicon crystal type solar cells”, “amorphoussilicon type solar cells” and “compound semiconductor type solar cells”,and the silicon crystal type solar cells further include “single crystaltype solar cells” and “polycrystal type solar cells”. Among these, thesolar cells showing high conversion efficiency, which is the mostimportant characteristic as a solar cell, are the “compoundsemiconductor type solar cells”, and the conversion efficiency thereofreaches almost 25%. However, as for the compound semiconductor typesolar cells, it is extremely difficult to produce compoundsemiconductors used as the materials thereof, thus they have a problemfor becoming popular for general use in respect of the production costof solar cell substrates, and use thereof has been limited.

[0005] The term “conversion efficiency” used herein means a valuerepresenting “a ratio of energy, which can be converted into electricenergy by a solar cell taken out of the solar cell, to energy of lightirradiated on the solar cell” and represented in percentage (%) (it isalso called photoelectric conversion efficiency).

[0006] Solar cells showing high conversion efficiency in the next placeto the compound semiconductor type solar cells are silicon singlecrystal type solar cells. Since they show power generation efficiency of20% order, which is close to that of the compound semiconductor solarcells, and substrates for those solar cells may be relatively easilyobtained, they constitute the mainstream of the solar cells for widegeneral use. Furthermore, the silicon polycrystal type solar cells andamorphous silicon type solar cells are also practically used because oflow production cost of the solar cell substrate materials therefor,although the conversion efficiencies thereof are inferior to those ofthe aforementioned two types of solar cells, i.e., about 5 to 15%.

[0007] A general method for producing a silicon single crystal typesolar cell will be briefly explained hereafter. First, a cylindricalingot of silicon single crystal is produced by the Czochralski method(referred to as the CZ method or the Czochralski method hereafter) orthe floating zone melting method (referred to as the FZ method or thefloating zone method hereafter) in order to obtain a silicon waferserving as a substrate of a solar cell. Further, this ingot is slicedinto, for example, a thin wafer having a thickness of about 300 μm, andthe wafer is etched for the surface with a chemical solution to removemechanical damages on the surface to obtain a wafer (substrate) used asa solar cell. This wafer is subjected to a diffusion treatment forimpurity (dopant) to form a pn-junction on one side of the wafer,electrodes are attached on the both sides, and an antireflection filmfor reducing light energy loss by light reflection is finally attachedon the sunlight incidence side surface to complete a solar cell.

[0008] Although demands for solar cells are recently increasing as oneof clean energy sources with a background of environmental problems, thehigher energy cost compared with general commercial powers hasconstituted an obstacle of wide use thereof. In order to reduce the costof silicon crystal solar cells, it is necessary to further increase theconversion efficiency as well as to reduce the production cost ofsubstrates. For this reason, the cost of substrate material has beenreduced by using cone portions, tail portions of single crystal ingotsand so forth, which can not be made into products or are not suitablefor electronic use for producing so-called semiconductor devices, as rawmaterials of substrates of single crystal type solar cells. However,such supply of raw materials is unreliable, and the amount thereof isalso limited. Therefore, considering expansion of the demands forsilicon single crystal type solar cells in future, it will be difficultto stably produce solar cell substrates in a required amount by such amethod.

[0009] Moreover, in solar cells, it is important to produce a solar cellof larger area in order to obtain a larger electric current. As a methodof obtaining a silicon wafer having a large diameter used as a substratematerial for producing a solar cell of large area, the CZ method issuitable, which enables easy production of a silicon single crystalhaving a large diameter and provides superior strength of a producedsingle crystal. Therefore, the CZ method constitutes the mainstream ofthe production of silicon crystals for solar cells.

[0010] Further, if a silicon wafer serving as a substrate material of asingle crystal type solar cell does not have a substrate lifetime(referred to as “lifetime” or abbreviated as LT hereafter), which is oneof the characteristics thereof, of 10 μs or more, it cannot be used as asolar cell substrate. Furthermore, in order to obtain a solar cell ofhigh conversion efficiency, it is required that the substrate lifetimeshould be preferably 200 μs or more.

[0011] However, as for a single crystal produced by the CZ method, whichis the mainstream of the current methods for producing single crystalingots, if the solar cell is irradiated with a strong light when thesingle crystal is processed into a solar cell, lifetime of the solarcell substrate is reduced, and photodegradation is caused. Therefore,sufficient conversion efficiency cannot be obtained, and improvement isdesired also for performance of solar cells.

[0012] It is known that the cause of the reduction of the lifetime andthe photodegradation upon irradiation of strong light on a solar cellproduced by using such a CZ method silicon single crystal is aninfluence of boron and oxygen present in the single crystal substrate.Currently, the conductivity type of wafers used as solar cells is mainlyp-type, and boron is usually added to p-type wafers as a dopant.Although a single crystal ingot used as the material of the wafer may beproduced by either the CZ method (including the magnetic field appliedCZ method, also referred to as the MCZ method hereinafter) or the FZmethod, the FZ method suffers from higher production cost for singlecrystal ingots compared with the CZ method, and in addition, a siliconsingle crystal having a large diameter is more easily produced by the CZmethod as described above. Therefore, at present, single crystals aremainly produced by the CZ method, which enables production of singlecrystals having a large diameter at a relatively low cost.

[0013] However, a crystal produced by the CZ method suffers from aproblem that it contains oxygen at a high concentration, and thus thelifetime characteristic is affected by boron and oxygen in a p-typeCZ-method silicon single crystal to cause photodegradation.

[0014] In order to solve such a problem, the applicants of the presentapplication proposed use of Ga (gallium) instead of B (boron) as ap-type doping agent in a previous application (PCT/00/02850). By usingGa as a dopant as described above, it became possible to preventreduction of the lifetime due to the influence of B and oxygen.

[0015] However, in spite of the elimination of the influences of B andoxygen by use of Ga as the dopant, the lifetime might be reduced andcharacteristics of solar cells fluctuated among produced solar cells.Such fluctuation of characteristics invited decrease of production yieldof solar cells and decrease of the conversion efficiency as the wholesolar cell module and thus caused a problem.

DISCLOSURE OF THE INVENTION

[0016] The present invention was accomplished in view of such a problem,and its object is, when a solar cell is produced by using a CZ siliconsingle crystal wafer, to provide a solar cell showing little fluctuationof characteristics by using a CZ silicon single crystal wafer that doesnot reduce the lifetime.

[0017] In order to solve the aforementioned problem, the presentinvention provides a method for producing a solar cell comprisingforming the solar cell from a CZ silicon single crystal wafer, wherein aCZ silicon single crystal wafer having an initial interstitial oxygenconcentration of 15 ppma or less is used as the silicon single crystalwafer.

[0018] If the initial interstitial oxygen concentration is 15 ppma(JEIDA: Japan Electronic Industry Development Association Standard) orless as described above, oxygen precipitation is hardly generated by aheat treatment for producing a solar cell, a solar cell that avoidsreduction of lifetime by BMD can be obtained, and thus a favorable solarcell showing little fluctuation of characteristics can be produced.

[0019] In the aforementioned method, the CZ silicon single crystal waferis preferably a p-type silicon single crystal wafer containing Ga as adopant.

[0020] By using Ga as a dopant of p-type silicon single crystal waferinstead of boron, the photodegradation caused by not only BMD but alsopresence of boron and oxygen can be prevented.

[0021] In the aforementioned method, the concentration of Ga is 3×10¹⁵to 5×10¹⁷ atoms/cm³.

[0022] If the concentration of Ga is 3×10¹⁵ atoms/cm³ or more asdescribed above, reduction of conversion efficiency by consumption ofpower due to increase of internal resistance of the solar cell can besuppressed, and if the concentration of Ga is 5×10¹⁷ atoms/cm³ or lessas described above, the so-called Auger recombination phenomenon, i.e.,reduction of the lifetime due to capture of minority carriers by Gaatoms, can be prevented.

[0023] Further, the solar cell produced by the method of the presentinvention is, for example, a solar cell produced from a CZ siliconsingle crystal wafer, wherein the CZ silicon single crystal wafer has aninterstitial oxygen concentration of 15 ppma or less.

[0024] If the solar cell is produced from a CZ silicon single crystalwafer having an interstitial oxygen concentration of 15 ppma or less asdescribed above, oxygen precipitation is hardly generated by a heattreatment for producing a solar cell, and the interstitial oxygenconcentration is not so high either. Therefore, the reduction of thelifetime resulting from oxygen atoms themselves can be suppressed, andthe solar cell shows little fluctuation of characteristics.

[0025] The solar cell of the present invention is also a solar cellproduced from a CZ silicon single crystal wafer, wherein the CZ siliconsingle crystal wafer has a BMD density of 5×10⁸/cm³ or less.

[0026] If the CZ silicon single crystal wafer has a BMD density of5×10⁸/cm³ or less as described above, sharp reduction of the lifetimecan be prevented, the conversion efficiency of the solar cell can alsobe maintained at a high level, and thus a solar cell showing littlefluctuation in characteristics can be provided.

[0027] In this case, the CZ silicon single crystal wafer constitutingthe aforementioned solar cell is preferably a p-type silicon singlecrystal wafer containing Ga as a dopant.

[0028] This is because, if the dopant of p-type silicon single crystalwafer is not boron, but Ga, as described above, the photodegradationcaused by the presence of boron and oxygen can also be prevented.

[0029] In this case, the concentration of Ga is preferably 3×10¹⁵ to5×10¹⁷ atoms/cm³.

[0030] If the concentration of Ga is 3×10¹⁵ atoms/cm³ or more asdescribed above, reduction of conversion efficiency by consumption ofpower due to increase of internal resistance of the solar cell can besuppressed, and if the concentration of Ga is 5×10¹⁷ atoms/cm³ or lessas described above, reduction of lifetime caused due to capture ofminority carriers by Ga atoms, i.e., the so-called Auger recombinationphenomenon, can be prevented.

[0031] As described above, according to the method for producing a solarcell and the solar cell of the present invention, a solar cell showinglittle fluctuation of characteristics can be obtained, and a solar cellof high efficiency can be obtained at a low cost.

BRIEF EXPLANATION OF THE DRAWINGS

[0032]FIG. 1 is a graph showing relationship between resistivity andlifetime for three kinds of Ga-doped silicon single crystal wafershaving different oxygen concentrations.

[0033]FIG. 2 is a graph showing relationship between BMD density andlifetime in a silicon single crystal wafer.

BEST MODE FOR CARRYING OUT THE INVENTION

[0034] The present invention will be further explained hereafter.

[0035] The inventors of the present invention conceived thatinterstitial oxygen and crystal defects such as oxide precipitates inthe wafer bulk (hereafter also referred to as BMD's (Bulk MicroDefects)) contained in a CZ silicon single crystal wafer from which asolar cell was produced might affect conversion efficiency of the solarcell, even if they did not coexist with boron, and assiduously conductedresearches. As a result, they accomplished the present invention.

[0036] In the case of CZ silicon single crystal, since a single crystalis pulled by using a crucible made of quartz, oxygen (interstitialoxygen) is inevitably incorporated into the pulled crystal. Theinterstitial oxygen concentration in a crystal immediately after thepulling (as-grown) or a CZ silicon single crystal wafer produced fromthe crystal is called initial interstitial oxygen concentration, and itis known that, if a CZ silicon single crystal wafer containing suchinterstitial oxygen is subjected to a heat treatment in a subsequentprocess, oxide precipitates will precipitate in the bulk.

[0037] In this case, photodegradation of conversion efficiency is causedby the presence of oxygen and boron as described above, and this problemis solved by changing the dopant from boron to Ga as described in theprevious application. However, use of Ga as a dopant did not alwayscause to reduce the lifetime, it might actually reduce the lifetimeamong the produced solar cells, and thus fluctuation in thecharacteristics of solar cells might be observed.

[0038] The inventors of the present invention estimated that the abovephenomenon was caused by a high concentration of interstitial oxygencontained in the silicon single crystal wafer. As for Ga, it is knownthat, if the Ga concentration becomes higher, minority carriers morefrequently approach Ga, and if they are captured by Ga, the so-calledAuger recombination phenomenon is caused, i.e., the lifetime is reduced.Although such an effect is not known for oxygen atoms, on the otherhand, it can be considered that oxygen atoms increase the latticeconstant of silicon and influence on the mobility of the minoritycarriers.

[0039] However, it was considered that, since no attention wasconventionally paid at all for the initial interstitial oxygenconcentration in a silicon single crystal wafer from which a solar cellwas formed or BMD in a silicon single crystal wafer constituting aproduced solar cell, the characteristics of solar cells fluctuated.

[0040] Therefore, the inventors of the present invention measured thelifetime with changing the oxygen concentration in silicon singlecrystal wafers doped with Ga. FIG. 1 is a graph showing the relationshipbetween resistivity, i.e., the doped amount of Ga, and the lifetime forthree kinds of Ga-doped silicon single crystal wafers having differentoxygen concentrations. As shown in FIG. 1, it can be seen that, even ifGa is doped at the same amount, a wafer having a higher oxygenconcentration shows a shorter lifetime.

[0041] Further, BMD's such as fine oxide precipitates may be generatedin the bulk of silicon single crystal wafer of solar cell by the process(heat treatment) for producing the solar cell. Since BMD functions as agettering site that captures heavy metal contamination and so forth,which are harmful in the production processes of semiconductor devicessuch as LSI, it constitutes one of the advantages of CZ silicon singlecrystal wafers.

[0042] However, the inventors of the present invention suspected thatsuch a BMD might adversely affect the lifetime of the minority carriers.Therefore, the inventors of the present invention investigated therelationship between the BMD density and the lifetime of the minoritycarriers in the bulk. FIG. 2 is a graph showing the relationship betweenthe BMD density and the lifetime of minority carriers in a siliconsingle crystal wafer. As a result of the measurement, it was found thatthe lifetime was sharply decreased, when the BMD density exceeded5×10⁸/cm³, as shown in FIG. 2. Therefore, it is expected that BMD'sremaining after the formation of solar cell are preferably reduced asmuch as possible so that their density should be reduced to a level of5×10⁸/cm³ or less, because a CZ silicon single crystal wafer having alonger lifetime is excellent in view of the conversion efficiency as aCZ silicon single crystal wafer used for the production of a solar cell.

[0043] These facts revealed that the interstitial oxygen in a siliconsingle crystal wafer from which a solar cell is produced influenceslifetime of a solar cell as interstitial oxygen itself or a fine oxideprecipitate irrespective of the presence or absence of boron as adopant.

[0044] Further, as for the relationship between the initial oxygenconcentration and the amount of precipitated oxygen varying depending onthe heat treatment conditions of a CZ silicon single crystal wafer, itis known that the oxygen precipitation is hardly caused if the initialoxygen concentration is about 15 ppma (JEIDA) or less (refer to TakaoAbe, Advanced Electronics Series I-5 “Silicon”, p.195, FIG. 8.1,published from Baifukan). Therefore, if a CZ silicon single crystalwafer has an initial oxygen concentration of 15 ppma or less, there arehardly generated oxide precipitates, which are harmful to the conversionefficiency, after the heat treatment, and such a wafer is preferred as aCZ silicon single crystal wafer used for the manufacture of a solarcell.

[0045] The present invention was accomplished based on such a principleas described above with investigations of various conditions.

[0046] The present invention will be explained in more detail hereafter.However, the present invention is not limited to these explanations.

[0047] In order to obtain an initial interstitial oxygen concentrationof 15 ppma or less falling within the range according to the presentinvention, a method conventionally used in the CZ method may be used. Aninitial interstitial oxygen concentration within the aforementionedrange can be easily obtained by means of, for example, reduction ofcrucible rotational speed, increase of introduced gas flow rate,reduction of atmospheric pressure, control of temperature distributionof silicon melt, control of convection and so forth. Further, whenpulling is performed with an oxygen concentration of about 10 ppma orless, the oxygen concentration can be lowered to about 7 ppma by usingthe so-called MCZ method, in which the pulling is performed withapplication of a magnetic field.

[0048] As for the general production process for a solar cell using asilicon single crystal wafer, the solar cell is produced mainly througha pn-junction formation step, an electrode formation step and anantireflection film formation step. In the pn-junction formation step, apn-junction is usually formed by introducing n-type impurities into ap-type silicon single crystal wafer surface, and the gas diffusionmethod, solid phase diffusion method, ion implantation method or thelike is used for the introduction of impurities at that time, in which aheat treatment is performed at a temperature from several hundredsdegrees ° C. to 1000° C. or higher. Further, the electrode formationstep is a step of forming metal serving as an electrode by vacuumevaporation, plating method, printing method or the like, and a heattreatment at about several hundreds degrees ° C. is performed.Furthermore, in the antireflection film formation step, a deposited filmis formed by the CVD (chemical vapor deposition) method, PVD (physicalvapor deposition) method or the like, and a heat treatment at severalhundreds degrees ° C. to about 800° C. is performed also in that case.

[0049] Which method is used, i.e., what kinds of heat treatmentconditions are used, in each of these steps is a matter depending onproduction conditions specific to each of the various solar cells.Therefore, in order to determine an initial oxygen concentration of asilicon single crystal wafer used for producing a solar cell having aBMD density of 5×10⁸/cm³ or less according to the present invention,heat treatment conditions in the production of each solar cell can bepreliminarily specified and relationship between the BMD densityexisting after the heat treatment and the initial oxygen concentrationof the silicon single crystal wafer used for producing a solar cell isexperimentally obtained to determine such an initial oxygenconcentration in the silicon single crystal wafer that the BMD densityshould become 5×10⁸/cm³ or less.

[0050] In order to form oxide precipitates in a silicon single crystalwafer by a heat treatment, it is necessary that oxygen should exist in asupersaturated state at the heat treatment temperature and fine oxideprecipitates, metal impurities or the like serving as nuclei forformation of precipitates (precipitation nuclei) should exist.

[0051] Although the precipitation nuclei also exist in an as-grownsilicon wafer, they are also formed when supersaturated oxygen becomefine oxide precipitates upon a heat treatment at a temperature of about650° C. to 900° C. In order for such precipitation nuclei to grow andbecome large oxide precipitates, a heat treatment at a temperature of900° C. to 1100° C. is required. A heat treatment at a temperature lowerthan 900° C. requires an extremely long period of time for the growthbecause of slow diffusion of oxygen, and thus large oxide precipitatesare hardly formed. At a temperature exceeding 1100° C., precipitates arenot formed since precipitation nuclei are remelted and thus eliminated.

[0052] Therefore, although the BMD density can be made very small if allof the heat treatments for forming a solar cell by using a CZ siliconsingle crystal wafer are performed at a temperature exceeding 1100° C.,a low temperature process at 1000° C. or lower is mainly used in anactual process as described above. That is, although oxide precipitatesof a large size are not formed when a solar cell is formed by such aprocess, it has been subjected to a heat treatment at such a temperaturethat at least fine oxide precipitates serving as precipitation nucleishould be formed, and therefore generation of such precipitation nucleiconstitute a factor of the reduction of lifetime.

[0053] On the other hand, if a high temperature of about 900° C. to1100° C. is employed as the heat treatment temperature for forming asolar cell, the size of oxide precipitates becomes large and thus thereduction of lifetime also becomes large. Therefore, the effect ofpreventing the reduction of lifetime becomes much more significant in asolar cell having a BMD density of 5×10⁸/cm³ or less according to thepresent invention, and thus it is more effective.

[0054] The BMD density of a formed solar cell can be measured by the OPP(optical precipitate profiler) method, or measured by cleaving the waferand preferentially etching the cleaved surface. If the size of BMD issmall, the oxide precipitates can be grown to a detectable size, forexample, by a heat treatment at 1000° C. for 16 hours and then thedensity can be measured.

[0055] The OPP method utilizes a differential interference microscope ofNomarski type, in which a laser light emitted from a light source isfirst separated into two linearly polarized light beams by a polarizingprism, which are orthogonally intersects with each other and have phasesdifferent from each other by 90°, and the beams are entered into a waferfrom the wafer mirror surface side. At that time, when one beam passes adefect, phase shift is caused and hence phase difference from anotherbeam is caused. This phase difference is detected by a polarizationanalyzer after the beams transmit the wafer back surface to detect thedefect.

[0056] Further, in order to prevent the photodegradation of a solarcell, Ga is preferably doped in an amount of 3×10¹⁵ to 5×10¹⁷ atoms/cm³(5 to 0.1 Ù·cm in terms of resistivity) as a p-type dopant of a siliconsingle crystal wafer. When the Ga concentration is lower than 3×10¹⁵atoms/cm³, resistivity of the wafer becomes higher than required, thuspower is consumed due to the internal resistance of the solar cell, andthe conversion efficiency may be reduced. Further, if the Gaconcentration is higher than 5×10¹⁷ atoms/cm³, resistivity of the waferis unduly reduced, and thus the lifetime of the minority carriers in thewafer may be reduced by the Auger recombination.

[0057] In order to produce a CZ silicon single crystal added with Ga,after Ga is added to silicon melt in a crucible, a seed crystal can bebrought into contact with the silicon melt and pulled with rotation. Inthis case, as for the addition of Ga to the melt in the crucible, asilicon crystal added with Ga at a high concentration can be grownbeforehand, and a doping agent produced by crushing the silicon crystaldoped with Ga at a high concentration can be added to the silicon meltin a calculated appropriate amount. Thus, an accurate amount of Ga canbe doped.

[0058] Hereafter, the present invention will be specifically explainedwith reference to the following examples and comparative example.However, the present invention is not limited to these.

EXAMPLES 1 TO 3, COMPARATIVE EXAMPLE 1

[0059] A silicon single crystal having an initial interstitial oxygenconcentration of about 14 ppma was pulled by the usual CZ method (dopedwith Ga, dopant concentration: about 1×10¹⁶ atoms/cm³, Example 1).Further, a silicon single crystal having an initial interstitial oxygenconcentration of about 10 ppma (doped with B, dopant concentration:about 1×10¹⁶ atoms/cm³, Example 2) and a silicon single crystal havingan initial interstitial oxygen concentration of about 8 ppma (doped withGa, dopant concentration: about 1×10¹⁶ atoms/cm³, Example 3) were pulledby the MCZ method.

[0060] Further, as a comparative example, a silicon single crystalhaving an initial interstitial oxygen concentration of about 20 ppma(doped with B, dopant concentration: about 1×10¹⁶ atoms/cm³) was pulledby the usual CZ method. Mirror polished wafers having a diameter of 150mm and crystal orientation of <100> were produced from those singlecrystals.

[0061] Lifetime of these four kinds of wafers was measured by using themicrowave-PCD method (photo conductive decay method). Then, withoutactually forming solar cells, the wafers were subjected to a heattreatment consisting of three stages of 800° C. for 30 minutes(pn-junction formation step)+600° C. for 30 minutes (electrode formationstep)+700° C. for 60 minute (antireflection film formation step) as aheat treatment simulating the production process of a solar cell, andthereafter the lifetime was measured again. Further, the wafers weresubjected to a heat treatment at 1000° C. for 16 hours so that BMD'sshould grow to have a detectable size, and the BMD density was measuredby the OPP method.

[0062] As a result, the wafers of Examples 1 to 3 all showed favorablevalues of lifetime, i.e., 500 microseconds or more before and after theheat treatment, and reduction of the lifetime due to the heat treatmentwas not observed. Further, the BMD density was 1×10⁸/cm³ or less for allof the wafers.

[0063] On the other hand, as for the wafer of Comparative Example 1,about 20% reduction of the lifetime due to the heat treatment wasobserved, and the BMD density was about 1×10⁹/cm³.

[0064] The present invention is not limited to the embodiments describedabove. The above-described embodiments are mere examples, and thosehaving the substantially same structure as that described in theappended claims and providing the similar functions and advantages areincluded in the scope of the present invention.

1. A method for producing a solar cell comprising forming the solar cellfrom a CZ silicon single crystal wafer, wherein a CZ silicon singlecrystal wafer having an initial interstitial oxygen concentration of 15ppma or less is used as the CZ silicon single crystal wafer.
 2. Themethod for producing a solar cell according to claim 1, wherein the CZsilicon single crystal wafer is a p-type silicon single crystal wafercontaining Ga as a dopant.
 3. The method for producing a solar cellaccording to claim 2, wherein the concentration of Ga is 3×10¹⁵ to5×10¹⁷ atoms/cm³.
 4. A solar cell produced by the method according toany one of claims 1 to
 3. 5. A solar cell produced from a CZ siliconsingle crystal wafer, wherein the CZ silicon single crystal wafer has aninterstitial oxygen concentration of 15 ppma or less.
 6. A solar cellproduced from a CZ silicon single crystal wafer, wherein the CZ siliconsingle crystal wafer has a BMD density of 5×10⁸/cm³ or less.
 7. Thesolar cell according to claim 5 or 6, wherein the CZ silicon singlecrystal wafer constituting the solar cell is a p-type silicon singlecrystal wafer containing Ga as a dopant.
 8. The solar cell according toclaim 7, wherein the concentration of Ga is 3×10¹⁵ to 5×10¹⁷ atoms/cm³.